This application claims priority to Austrian application No. A1739/98, filed Oct. 19, 1998, herein incorporated by reference.
1. Field of the Invention
The invention relates to aqueous self-crosslinking copolymer dispersions having a core-shell structure, at least some of the monomers forming the copolymer in the core having carbonyl groups and carboxyl groups as well as free-radically polymerizable olefinically unsaturated groups, and comprising dicarboxylic dihydrazides or bishydrazine compounds. The invention additionally relates to a two-stage process for preparing these aqueous self-crosslinking copolymer dispersions and to their use as binders for storage-stable one-component coating materials which crosslink at room temperature or at elevated temperature.
2. Description of the Related Art
EP-A 0 649 865 discloses self-crosslinking carbonyl-containing polyurethane-vinyl hybrid dispersions which crosslink by way of hydrazine compounds but do not have a core-shell structure.
EP-A 0 555 774 discloses copolymer dispersions formed by copolymerizing monomers having carbonyl groups and carboxyl groups. These dispersions also do not have a core-shell structure, as they are prepared in a single-stage polymerization with no change in the composition of the monomer mixture. The binders prepared with these dispersions are crosslinked by reaction with the polyamines employed as neutralizing agents. With these products, first a high proportion of carbonyl groups is required to achieve sufficient crosslinking density of the coating films and second this high proportion of corresponding monomers results, during the copolymerization, in the formation of gel specks and coagulum whose removal can only be accomplished at great expense in terms of time and apparatus. In addition, the polyamine employed as crosslinking component leads to severe yellowing of the coating film and so to problems in numerous applications.
AT-B 402 504 describes self-crosslinking aqueous polymer dispersions which are prepared in a two-stage process. In this process, the composition of the monomer mixture for the second stage of the polymerization differs from that of the first stage. These copolymer dispersions have small fractions of carbonyl groups and permit high crosslinking densities without forming the coagulum or gel specks that are difficult to remove. However, the polyamines employed as crosslinking component likewise occasion severe yellowing of the resulting coating films.
European Patent Application EP-A 0 795 568 describes aqueous polymer dispersions having a core-shell structure in the core of which at least 50% of the monomers which are employed are those whose homopolymers have a glass temperature of below 0xc2x0 C. while the proportion of such monomers employed in the shell is less than 45%. Difunctional acid hydrazides can also be added to the dispersion. Where such hydrazides are added in the examples, the monomer mixture for the shell of the polymer particles also includes a monomer that contains ketone groups. In this case, the monomer composition is selected in accordance with the desired minimum film-forming temperatures.
In AT-B 403 478, the dispersions known from AT-B 402 504 are combined with polyfunctional isocyanates of low molar mass to form two-component binders. Although it is possible in this way to provide coating materials which have a reduced tendency to yellow and an improved resistance to water, chemicals and light exposure, there are disadvantages associated with these coating materials. These include the complex preparation of the coating material, the limited pot life, prolonged drying time, and relatively low film hardness.
It has surprisingly now been found that aqueous copolymer dispersions prepared in accordance with AT-B 402 504 by a two-stage process in which monomers having in each case carbonyl and carboxyl groups are employed only in the first stage (which leads to the core of the particles) can be crosslinked, following neutralization of the carboxyl groups by adding small amounts of ammonia, amines or aqueous alkalis, with polyfunctional hydrazides and lead to binders having a sufficient pot life which can be processed to give coating materials having high film hardness, good resistance to mechanical and chemical exposure, rapid drying, and little or no tendency to yellow.
The invention accordingly provides aqueous self-crosslinking copolymer dispersions ABC obtainable by free-radically initiated copolymerization in the first stage of a monomer mixture A comprising mass fractions in the mixture of
A1 from about 2 to about 55%, preferably from about 4 to about 45% and, with particular preference, from about 6 to about 40% of olefinically unsaturated monomers having in each case at least one carbonyl group per molecule,
A2 from about 0.5 to about 20%, preferably from about 1 to about 15% and, with particular preference, from about 3 to about 12% of xcex1,xcex2-olefinically unsaturated carboxylic acids or monoesters of xcex1,xcex2-olefinically unsaturated dicarboxylic acids with linear, branched or cyclic alcohols having 1 to 15 carbon atoms,
A3 from about 20 to about 70%, preferably from about 22 to about 60% and, with particular preference, from about 24 to about 50% of olefinically unsaturated monomers selected from vinylaromatic compounds, n-butyl methacrylate and also alkyl esters of xcex1,xcex2-olefinically unsaturated carboxylic acids or dialkyl esters of xcex1,xcex2-olefinically unsaturated dicarboxylic acids, the alkyl groups being selected from linear and branched alkyl groups having up to 3 carbon atoms in the alkyl radical and cyclic and polycyclic alkyl groups having 5 to 15 carbon atoms in the alkyl group,
A4 from about 10 to about 60%, preferably from about 15 to about 50% and, with particular preference, from about 20 to about 45% of esters selected from alkyl esters of xcex1,xcex2-olefinically unsaturated carboxylic acids or dialkyl esters of xcex1,xcex2-olefinically unsaturated dicarboxylic acids, the alkyl groups being selected from linear and branched alkyl groups having more than 3 carbon atoms in the alkyl radical, with the exception of n-butyl methacrylate, and
A5 from 0 to about 25%, preferably from about 2 to about 20% and, with particular preference, from about 5 to about 15% of other free-radically polymerizable monomers selected from vinyl esters of aliphatic saturated carboxylic acids having 2 to 18 carbon atoms, hydroxyalkyl esters, nitrites and amides of xcex1,xcex2-unsaturated carboxylic acids,
the sum of the mass fractions of components A1 to A5 necessarily being 100%, and by subsequent addition of a second monomer mixture B and further free-radically initiated polymerization of this monomer mixture in the second stage, the mixture B comprising mass fractions of
B1 from about 30 to about 90%, preferably from about 40 to about 80% and, with particular preference, from about 50 to about 75% of olefinically unsaturated monomers selected from vinylaromatic compounds, n-butyl methacrylate and also alkyl esters of xcex1,xcex2-olefinically unsaturated carboxylic acids or dialkyl esters of xcex1,xcex2-olefinically unsaturated dicarboxylic acids, the alkyl groups being selected from linear and branched alkyl groups having up to 3 carbon atoms in the alkyl radical and cyclic and polycyclic alkyl groups having 5 to 15 carbon atoms in the alkyl group,
B2 from about 10 to about 60%, preferably from about 20 to about 50% and, with particular preference, from about 25 to about 40% of esters selected from alkyl esters of xcex1,xcex2-olefinically unsaturated carboxylic acids or dialkyl esters of xcex1,xcex2-olefinically unsaturated dicarboxylic acids, the alkyl groups being selected from linear and branched alkyl groups having more than 3 carbon atoms in the alkyl radical, with the exception of n-butyl methacrylate, and
B3 from 0 to about 40%, preferably from about 5 to about 30% and, with particular preference, from about 10 to about 25% of other free-radically polymerizable monomers selected from vinyl esters of aliphatic saturated carboxylic acids having 2 to 18 carbon atoms, hydroxyalkyl esters, nitrites and amides of xcex1,xcex2-unsaturated carboxylic acids,
the sum of the mass fractions of components B1 to B3 necessarily being 100%, and the ratio of the mass of the monomer mixture A to the mass of the monomer mixture B is from about 50:50 to about 95:5, preferably from about 60:40 to about 90:10.
The copolymers AB obtainable in this way preferably contain, based on the mass of the solids of the dispersion, from about 0.2 to about 1.7 mol/kg of carbonyl groups and from about 0.15 to about 1.6 mol/kg of carboxyl groups. The amount of substance of the carboxyl groups here is always lower than the amount of substance of the carbonyl groups; preferably, the ratio of the amount of substance of the carboxyl groups n(xe2x80x94COOH) to the amount of substance of the carbonyl groups n( greater than CO) is from about 0.5 to about 0.95 mol/mol, with particular preference from about 0.75 to about 0.9 mol/mol.
The resulting dispersions of the copolymers AB are neutralized following the polymerization by adding ammonia, amines or aqueous alkalis, the amount in which the neutralizing agents are added being such that it is sufficient to neutralize from 0 to about 120% of the carboxyl groups present.
To the dispersion neutralized in this way there are subsequently added compounds C having at least two hydrazine or hydrazide groups per molecule, in an amount such that the ratio r of the sum of the amount of substance of hydrazine groups n(xe2x80x94NHxe2x80x94NH2) and the amount of substance of hydrazide groups n(xe2x80x94COxe2x80x94NHxe2x80x94NH2) to the amount of substance of the carbonyl groups n( greater than CO) is from about 0.5 to about 1.1 mol/mol, preferably between about 0.8 and about 1.0 mol/mol.
The invention additionally provides a process for preparing the copolymer dispersions of the invention, in which water with or without emulsifiers is introduced as initial charge and heated to the desired reaction temperature, a mixture comprising water, emulsifiers if desired, the monomer mixture A, and a free-radical polymerization initiator is metered in to this initial charge, the polymerization is continued until at least 95% of the monomers have reacted, then a second mixture comprising water, emulsifiers if desired, and also the monomer mixture B and a further free radical forming polymerization initiator is metered in and the polymerization is subsequently continued until the residual monomer content has fallen below 1%. The resulting dispersion is cooled and admixed, with stirring, with the neutralizing agent, preferably in the form of an aqueous solution. Subsequently, the hydrazine compound or hydrazide compound C is added with stirring to the neutralized dispersion of the copolymer AB to form the self-crosslinking dispersion ABC.
Finally, the invention also provides for the use of these dispersions as binders for storage-stable one-component coating materials which crosslink at room temperature or at elevated temperature.
The amines or alkalis used if desired for neutralization improve the stability of the aqueous copolymer dispersions.
Olefinically unsaturated monomers containing carbonyl groups are employed as monomer component A1. Preference is given to the use of linear, branched and cyclic aliphatic compounds having 4 to 20 carbon atoms and each containing at least one carbonyl group and one ethylenic double bond. To a minor extent (up to 10% of the mass of the monomers A1) it is also possible to employ compounds having two or more polymerizable double bonds, which leads to crosslinking of the copolymer. Particular preference is given to the use of xcex1,xcex2-olefinically unsaturated monomers such as N-diacetone(meth)acrylamide and (meth)acrylic acid acetoacetoxyalkyl esters whose alkylene group is selected from 1,2-ethylene, 1,2- and 1,3-propylene, 1,4-butylene and 1,6-hexylene and also 1,5-(3-oxa)pentylene and 1,8-(3,6-dioxa) octylene groups, especially the ethyl esters.
The monomers A2 are preferably xcex1,xcex2-unsaturated carboxylic acids having 3 to 4 carbon atoms, such as acrylic acid, methacrylic acid, crotonic and isocrotonic acid and also vinylacetic acid. Monoesters of xcex1,xcex2-unsaturated carboxylic acids with linear, branched or cyclic alcohols having 1 to 15 carbon atoms can also be employed, the dicarboxylic acids preferably having 4 to 6 carbon atoms. Preference is given to monomethyl and monoethyl esters of maleic acid, fumaric acid, and also citraconic, mesaconic, itaconic and glutaconic acid.
Of the monomers A3, preference is given to the esters of methanol, ethanol, n-and isopropanol with acrylic and methacrylic acid and to the diesters of said alcohols with the dicarboxylic acids specified under A2, and also to n-butyl methacrylate and, of the vinylaromatic compounds, especially styrene, p-methylstyrene and the isomer mixture known as xe2x80x9cvinyltoluenexe2x80x9d. Particular preference is given to methyl and ethyl (meth)acrylate, n-butyl methacrylate, styrene, and dimethyl maleate.
As monomers A4 it is preferred to employ the esters of n-, sec-and tert-butanol, the isomeric pentanols and the higher alcohols such as n-hexanol and 2-ethylhexyl alcohol with xcex1,xcex2-unsaturated carboxylic acids selected from acrylic and methacrylic acid, vinylacetic acid, maleic acid and fumaric acid; in the case of the dicarboxylic acids referred to, the diesters are used, and n-butyl methacrylate is excluded.
As examples of the hydroxyalkyl esters of olefinically unsaturated carboxylic acids as monomer component A5 mention may be made of hydroxyethyl and 2-hydroxypropyl (meth)acrylate, (meth)acrylonitrile, (meth)acrylamide and, for the vinyl esters, of vinyl acetate.
In the second stage a polymer having a different chemical structure is produced by adding the monomer mixture B alone or together with new initiators.
The monomers B1 correspond to those specified under A3, the monomers B2 to those specified under A4, and the monomers B3, finally, to those specified under A5. The monomer mixture B is therefore free from compounds having functional groups selected from carbonyl and carboxyl groups.
The compounds C with hydrazine or hydrazide functionality comprise two or more hydrazine or hydrazide groups and preferably have an average molar mass (Mn) of less than about 1000 g/mol. Examples of such compounds are bishydrazides of dicarboxylic acids having 2 to 12 carbon atoms such as the bishydrazides of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pinalic acid, suberic acid, azelic acid, sebacic acid or the isomeric phthalic acids; carbonic bishydrazide, alkylene- or cycloalkylene-bissemicarbazides, N,Nxe2x80x2-diaminoguanidine, alkylene-bishydrazines such as N,Nxe2x80x2-diaminopiperazine, arylenebishydrazines such as phenylene- or naphthylenebishydrazine, and alkylenebissemicarbazides. Compounds C of higher functionality are, for example, the hydrazides of nitrilotriacetic acid or of ethylenediaminetetracetic acid.
The copolymer dispersions of the invention have excellent stability on storage and produce films having good resistance properties. They can be employed as binders for industrial coatings and primers. In addition, they can be used to formulate transparent coating materials which can be used in turn to coat wood, plastic, leather or paper.
The crosslinking of the copolymer dispersions takes place by reaction of the hydrazine or hydrazide groups of the compounds C with the carbonyl groups of the copolymer dispersions AB at temperatures as low as room temperature following the evaporation of a substantial proportion of water.
The formulation of water-dilutable coating materials using the binders prepared in accordance with the invention, and the application of such coating materials, are known to the skilled worker.
The examples which follow illustrate the invention without restricting it in its scope. In the examples below, as in the text which precedes them, all figures with the unit xe2x80x9c%xe2x80x9d are mass fractions unless specified otherwise. xe2x80x9cPartsxe2x80x9d are always parts by mass. Concentration figures in xe2x80x9c%xe2x80x9d are mass fractions of the dissolved substance in the solution (in g/100 g)).